Add `AnimationStyle` to `showSnackBar` (#142825)

fixes [`showSnackBar` is always replacing `animation` parameter of `SnackBar`](https://github.com/flutter/flutter/issues/141646)

### Code sample preview

![Screenshot 2024-02-02 at 21 10 57](https://github.com/flutter/flutter/assets/48603081/66d808f0-d638-4561-b9a4-96d1b93938f4)

examples/api/lib/material/scaffold/scaffold_messenger_state.show_snack_bar.2.dart

+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+
+/// Flutter code sample for [SnackBar].
+
+void main() => runApp(const SnackBarApp());
+
+class SnackBarApp extends StatelessWidget {
+  const SnackBarApp({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return const MaterialApp(
+      home: SnackBarExample(),
+    );
+  }
+}
+
+enum AnimationStyles { defaultStyle, custom, none }
+const List<(AnimationStyles, String)> animationStyleSegments = <(AnimationStyles, String)>[
+  (AnimationStyles.defaultStyle, 'Default'),
+  (AnimationStyles.custom, 'Custom'),
+  (AnimationStyles.none, 'None'),
+];
+
+class SnackBarExample extends StatefulWidget {
+  const SnackBarExample({super.key});
+
+  @override
+  State<SnackBarExample> createState() => _SnackBarExampleState();
+}
+
+class _SnackBarExampleState extends State<SnackBarExample> {
+  Set<AnimationStyles> _animationStyleSelection = <AnimationStyles>{AnimationStyles.defaultStyle};
+  AnimationStyle? _animationStyle;
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(title: const Text('SnackBar Sample')),
+      body: Center(
+        child: Column(
+          mainAxisAlignment: MainAxisAlignment.center,
+          children: <Widget>[
+            SegmentedButton<AnimationStyles>(
+              selected: _animationStyleSelection,
+              onSelectionChanged: (Set<AnimationStyles> styles) {
+                setState(() {
+                  _animationStyle = switch (styles.first) {
+                    AnimationStyles.defaultStyle => null,
+                    AnimationStyles.custom => AnimationStyle(
+                      duration: const Duration(seconds: 3),
+                      reverseDuration: const Duration(seconds: 1),
+                    ),
+                    AnimationStyles.none => AnimationStyle.noAnimation,
+                  };
+                });
+              },
+              segments: animationStyleSegments
+                .map<ButtonSegment<AnimationStyles>>(((AnimationStyles, String) shirt) {
+                  return ButtonSegment<AnimationStyles>(value: shirt.$1, label: Text(shirt.$2));
+                })
+                .toList(),
+            ),
+            const SizedBox(height: 10),
+            Builder(
+              builder: (BuildContext context) {
+                return ElevatedButton(
+                  onPressed: () {
+                    ScaffoldMessenger.of(context).showSnackBar(
+                      const SnackBar(
+                        content: Text('I am a snack bar.'),
+                        showCloseIcon: true,
+                      ),
+                      snackBarAnimationStyle: _animationStyle,
+                    );
+                  },
+                  child: const Text('Show SnackBar'),
+                );
+              }
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}

examples/api/test/material/scaffold/scaffold_messenger_state.show_snack_bar.2_test.dart

+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+import 'package:flutter_api_samples/material/scaffold/scaffold_messenger_state.show_snack_bar.2.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('ScaffoldMessenger showSnackBar animation can be customized using AnimationStyle',
+    (WidgetTester tester) async {
+      await tester.pumpWidget(
+        const example.SnackBarApp(),
+      );
+
+      // Tap the button to show the SnackBar with default animation style.
+      await tester.tap(find.byType(ElevatedButton));
+      await tester.pump();
+      await tester.pump(const Duration(milliseconds: 125)); // Advance the animation by 125ms.
+
+      expect(tester.getTopLeft(find.text('I am a snack bar.')).dy, closeTo(576.7, 0.1));
+
+      await tester.pump(const Duration(milliseconds: 125)); // Advance the animation by 125ms.
+
+      expect(tester.getTopLeft(find.text('I am a snack bar.')).dy, closeTo(566, 0.1));
+
+      // Tap the close button to dismiss the SnackBar.
+      await tester.tap(find.byType(IconButton));
+      await tester.pump();
+      await tester.pump(const Duration(milliseconds: 250)); // Advance the animation by 250ms.
+
+      expect(tester.getTopLeft(find.text('I am a snack bar.')).dy, closeTo(614, 0.1));
+
+      // Select custom animation style.
+      await tester.tap(find.text('Custom'));
+      await tester.pumpAndSettle();
+
+      // Tap the button to show the SnackBar with custom animation style.
+      await tester.tap(find.byType(ElevatedButton));
+      await tester.pump();
+      await tester.pump(const Duration(milliseconds: 1500)); // Advance the animation by 125ms.
+
+      expect(tester.getTopLeft(find.text('I am a snack bar.')).dy, closeTo(576.7, 0.1));
+
+      await tester.pump(const Duration(milliseconds: 1500)); // Advance the animation by 125ms.
+
+      expect(tester.getTopLeft(find.text('I am a snack bar.')).dy, closeTo(566, 0.1));
+
+      // Tap the close button to dismiss the SnackBar.
+      await tester.tap(find.byType(IconButton));
+      await tester.pump();
+      await tester.pump(const Duration(seconds: 1)); // Advance the animation by 1sec.
+
+      expect(tester.getTopLeft(find.text('I am a snack bar.')).dy, closeTo(614, 0.1));
+
+      // Select no animation style.
+      await tester.tap(find.text('None'));
+      await tester.pumpAndSettle();
+
+      // Tap the button to show the SnackBar with no animation style.
+      await tester.tap(find.byType(ElevatedButton));
+      await tester.pump();
+
+      expect(tester.getTopLeft(find.text('I am a snack bar.')).dy, closeTo(566, 0.1));
+
+      // Tap the close button to dismiss the SnackBar.
+      await tester.tap(find.byType(IconButton));
+      await tester.pump();
+
+      expect(find.text('I am a snack bar.'), findsNothing);
+  });
+}

packages/flutter/lib/src/material/scaffold.dart

@@ -290,13 +290,38 @@ class ScaffoldMessengerState extends State<ScaffoldMessenger> with TickerProvide
   /// ** See code in examples/api/lib/material/scaffold/scaffold_messenger_state.show_snack_bar.1.dart **
   /// {@end-tool}
   ///
-  ScaffoldFeatureController<SnackBar, SnackBarClosedReason> showSnackBar(SnackBar snackBar) {
+  /// If [AnimationStyle.duration] is provided in the [snackBarAnimationStyle]
+  /// parameter, it will be used to override the snackbar show animation duration.
+  /// Otherwise, defaults to 250ms.
+  ///
+  /// If [AnimationStyle.reverseDuration] is provided in the [snackBarAnimationStyle]
+  /// parameter, it will be used to override the snackbar hide animation duration.
+  /// Otherwise, defaults to 250ms.
+  ///
+  /// To disable the snackbar animation, use [AnimationStyle.noAnimation].
+  ///
+  /// {@tool dartpad}
+  /// This sample showcases how to override [SnackBar] show and hide animation
+  /// duration using [AnimationStyle] in [ScaffoldMessengerState.showSnackBar].
+  ///
+  /// ** See code in examples/api/lib/material/scaffold/scaffold_messenger_state.show_snack_bar.2.dart **
+  /// {@end-tool}
+  ///
+  ScaffoldFeatureController<SnackBar, SnackBarClosedReason> showSnackBar(
+    SnackBar snackBar,
+    { AnimationStyle? snackBarAnimationStyle }
+  ) {
     assert(
       _scaffolds.isNotEmpty,
       'ScaffoldMessenger.showSnackBar was called, but there are currently no '
       'descendant Scaffolds to present to.',
     );
-    _snackBarController ??= SnackBar.createAnimationController(vsync: this)
+    _didUpdateAnimationStyle(snackBarAnimationStyle);
+    _snackBarController ??= SnackBar.createAnimationController(
+        duration: snackBarAnimationStyle?.duration,
+        reverseDuration: snackBarAnimationStyle?.reverseDuration,
+        vsync: this,
+      )
       ..addStatusListener(_handleSnackBarStatusChanged);
     if (_snackBars.isEmpty) {
       assert(_snackBarController!.isDismissed);
@@ -355,6 +380,16 @@ class ScaffoldMessengerState extends State<ScaffoldMessenger> with TickerProvide
     return controller;
   }
 
+  void _didUpdateAnimationStyle(AnimationStyle? snackBarAnimationStyle) {
+    if (snackBarAnimationStyle != null) {
+      if (_snackBarController?.duration != snackBarAnimationStyle.duration ||
+          _snackBarController?.reverseDuration != snackBarAnimationStyle.reverseDuration) {
+        _snackBarController?.dispose();
+        _snackBarController = null;
+      }
+    }
+  }
+
   void _handleSnackBarStatusChanged(AnimationStatus status) {
     switch (status) {
       case AnimationStatus.dismissed:

packages/flutter/lib/src/material/snack_bar.dart

@@ -476,9 +476,14 @@ class SnackBar extends StatefulWidget {
   // API for ScaffoldMessengerState.showSnackBar():
 
   /// Creates an animation controller useful for driving a snack bar's entrance and exit animation.
-  static AnimationController createAnimationController({ required TickerProvider vsync }) {
+  static AnimationController createAnimationController({
+    required TickerProvider vsync,
+    Duration? duration,
+    Duration? reverseDuration,
+  }) {
     return AnimationController(
-      duration: _snackBarTransitionDuration,
+      duration: duration ?? _snackBarTransitionDuration,
+      reverseDuration: reverseDuration,
       debugLabel: 'SnackBar',
       vsync: vsync,
     );

packages/flutter/test/material/scaffold_test.dart

@@ -2868,6 +2868,241 @@ void main() {
       expect(tester.takeException(), isNull);
   });
 
+   testWidgets('ScaffoldMessenger showSnackBar default animatiom', (WidgetTester tester) async {
+    await tester.pumpWidget(MaterialApp(
+      home: Scaffold(
+        body: Builder(
+          builder: (BuildContext context) {
+            return ElevatedButton(
+              onPressed: () {
+                ScaffoldMessenger.of(context).showSnackBar(
+                  const SnackBar(
+                    content: Text('I am a snack bar.'),
+                    showCloseIcon: true,
+                  ),
+                );
+              },
+              child: const Text('Show SnackBar'),
+            );
+          },
+        ),
+      ),
+    ));
+
+    // Tap the button to show the SnackBar.
+    await tester.tap(find.byType(ElevatedButton));
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 125)); // Advance the animation by 125ms.
+
+    // The SnackBar is partially visible.
+    expect(tester.getTopLeft(find.text('I am a snack bar.')).dy, closeTo(576.7, 0.1));
+
+    await tester.pump(const Duration(milliseconds: 125)); // Advance the animation by 125ms.
+
+    // The SnackBar is fully visible.
+    expect(tester.getTopLeft(find.text('I am a snack bar.')).dy, closeTo(566, 0.1));
+
+    // Tap the close button to dismiss the SnackBar.
+    await tester.tap(find.byType(IconButton));
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 125)); // Advance the animation by 125ms.
+
+    // The SnackBar is partially visible.
+    expect(tester.getTopLeft(find.text('I am a snack bar.')).dy, closeTo(576.7, 0.1));
+
+    await tester.pump(const Duration(milliseconds: 125)); // Advance the animation by 125ms.
+
+    // The SnackBar is dismissed.
+    expect(tester.getTopLeft(find.text('I am a snack bar.')).dy, closeTo(614, 0.1));
+  });
+
+  testWidgets('ScaffoldMessenger showSnackBar animation can be customized', (WidgetTester tester) async {
+    await tester.pumpWidget(MaterialApp(
+      home: Scaffold(
+        body: Builder(
+          builder: (BuildContext context) {
+            return ElevatedButton(
+              onPressed: () {
+                ScaffoldMessenger.of(context).showSnackBar(
+                  const SnackBar(
+                    content: Text('I am a snack bar.'),
+                    showCloseIcon: true,
+                  ),
+                  snackBarAnimationStyle: AnimationStyle(
+                    duration: const Duration(milliseconds: 1200),
+                    reverseDuration: const Duration(milliseconds: 600),
+                  ),
+                );
+              },
+              child: const Text('Show SnackBar'),
+            );
+          },
+        ),
+      ),
+    ));
+
+    // Tap the button to show the SnackBar.
+    await tester.tap(find.byType(ElevatedButton));
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 300)); // Advance the animation by 300ms.
+
+    // The SnackBar is partially visible.
+    expect(tester.getTopLeft(find.text('I am a snack bar.')).dy, closeTo(602.6, 0.1));
+
+    await tester.pump(const Duration(milliseconds: 300)); // Advance the animation by 300ms.
+
+    // The SnackBar is partially visible.
+    expect(tester.getTopLeft(find.text('I am a snack bar.')).dy, closeTo(576.7, 0.1));
+
+    await tester.pump(const Duration(milliseconds: 600)); // Advance the animation by 600ms.
+
+    // The SnackBar is fully visible.
+    expect(tester.getTopLeft(find.text('I am a snack bar.')).dy, closeTo(566, 0.1));
+
+    // Tap the close button to dismiss the SnackBar.
+    await tester.tap(find.byType(IconButton));
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 300)); // Advance the animation by 300ns.
+
+    // The SnackBar is partially visible.
+    expect(tester.getTopLeft(find.text('I am a snack bar.')).dy, closeTo(576.7, 0.1));
+
+    await tester.pump(const Duration(milliseconds: 300)); // Advance the animation by 300ms.
+
+    // The SnackBar is dismissed.
+    expect(tester.getTopLeft(find.text('I am a snack bar.')).dy, closeTo(614, 0.1));
+  });
+
+  testWidgets('Updated snackBarAnimationStyle updates snack bar animation', (WidgetTester tester) async {
+    Widget buildSnackBar(AnimationStyle snackBarAnimationStyle) {
+      return MaterialApp(
+        home: Scaffold(
+          body: Builder(
+            builder: (BuildContext context) {
+              return ElevatedButton(
+                onPressed: () {
+                  ScaffoldMessenger.of(context).showSnackBar(
+                    const SnackBar(
+                      content: Text('I am a snack bar.'),
+                      showCloseIcon: true,
+                    ),
+                    snackBarAnimationStyle: snackBarAnimationStyle,
+                  );
+                },
+                child: const Text('Show SnackBar'),
+              );
+            },
+          ),
+        ),
+      );
+    }
+
+    // Test custom animation style.
+    await tester.pumpWidget(buildSnackBar(AnimationStyle(
+      duration: const Duration(milliseconds: 800),
+      reverseDuration: const Duration(milliseconds: 400),
+    )));
+
+    // Tap the button to show the SnackBar.
+    await tester.tap(find.byType(ElevatedButton));
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 400)); // Advance the animation by 400ms.
+
+    // The SnackBar is partially visible.
+    expect(tester.getTopLeft(find.text('I am a snack bar.')).dy, closeTo(576.7, 0.1));
+
+    await tester.pump(const Duration(milliseconds: 400)); // Advance the animation by 400ms.
+
+    // The SnackBar is fully visible.
+    expect(tester.getTopLeft(find.text('I am a snack bar.')).dy, closeTo(566, 0.1));
+
+    // Tap the close button to dismiss the SnackBar.
+    await tester.tap(find.byType(IconButton));
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 400)); // Advance the animation by 400ms.
+
+    // The SnackBar is dismissed.
+    expect(tester.getTopLeft(find.text('I am a snack bar.')).dy, closeTo(614, 0.1));
+
+    // Test no animation style.
+    await tester.pumpWidget(buildSnackBar(AnimationStyle.noAnimation));
+    await tester.pumpAndSettle();
+
+    // Tap the button to show the SnackBar.
+    await tester.tap(find.byType(ElevatedButton));
+    await tester.pump();
+
+    // The SnackBar is fully visible.
+    expect(tester.getTopLeft(find.text('I am a snack bar.')).dy, closeTo(566, 0.1));
+
+    // Tap the close button to dismiss the SnackBar.
+    await tester.tap(find.byType(IconButton));
+    await tester.pump();
+
+    // The SnackBar is dismissed.
+    expect(find.text('I am a snack bar.'), findsNothing);
+  });
+
+  testWidgets('snackBarAnimationStyle with only reverseDuration uses default forward duration',
+    (WidgetTester tester) async {
+      Widget buildSnackBar(AnimationStyle snackBarAnimationStyle) {
+        return MaterialApp(
+          home: Scaffold(
+            body: Builder(
+              builder: (BuildContext context) {
+                return ElevatedButton(
+                  onPressed: () {
+                    ScaffoldMessenger.of(context).showSnackBar(
+                      const SnackBar(
+                        content: Text('I am a snack bar.'),
+                        showCloseIcon: true,
+                      ),
+                      snackBarAnimationStyle: snackBarAnimationStyle,
+                    );
+                  },
+                  child: const Text('Show SnackBar'),
+                );
+              },
+            ),
+          ),
+        );
+      }
+
+      // Test custom animation style with only reverseDuration.
+      await tester.pumpWidget(buildSnackBar(AnimationStyle(
+        reverseDuration: const Duration(milliseconds: 400),
+      )));
+
+      // Tap the button to show the SnackBar.
+      await tester.tap(find.byType(ElevatedButton));
+      await tester.pump();
+      // Advance the animation by 1/2 of the default forward duration.
+      await tester.pump(const Duration(milliseconds: 125));
+
+      // The SnackBar is partially visible.
+      expect(tester.getTopLeft(find.text('I am a snack bar.')).dy, closeTo(576.7, 0.1));
+
+      // Advance the animation by 1/2 of the default forward duration.
+      await tester.pump(const Duration(milliseconds: 125)); // Advance the animation by 125ms.
+
+      // The SnackBar is fully visible.
+      expect(tester.getTopLeft(find.text('I am a snack bar.')).dy, closeTo(566, 0.1));
+
+      // Tap the close button to dismiss the SnackBar.
+      await tester.tap(find.byType(IconButton));
+      await tester.pump();
+      // Advance the animation by 1/2 of the reverse duration.
+      await tester.pump(const Duration(milliseconds: 200));
+
+      // The SnackBar is partially visible.
+      expect(tester.getTopLeft(find.text('I am a snack bar.')).dy, closeTo(576.7, 0.1));
+
+      // Advance the animation by 1/2 of the reverse duration.
+      await tester.pump(const Duration(milliseconds: 200)); // Advance the animation by 200ms.
+
+      // The SnackBar is dismissed.
+      expect(tester.getTopLeft(find.text('I am a snack bar.')).dy, closeTo(614, 0.1));
+  });
 }
 
 class _GeometryListener extends StatefulWidget {